1. Field of the Invention
The present invention relates to methods and compositions for treating subterranean formations, and more specifically, to methods and compositions for breaking fracturing fluids utilized in the stimulation of subterranean formations.
2. Description of the Prior Art
It is common practice to treat subterranean formations to increase the permeability or conductivity of such formations by procedures which are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well in order to produce one or more cracks or "fractures" in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells which are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a hydraulic fracturing fluid into the well and applying sufficient pressure on the fracturing fluid to cause the formation to break down with the attendant production of one or more fractures. The fracture or fractures may be horizontal or vertical, with the latter usually predominating, and with the tendency toward vertical fracture orientation increasing with the depth of the formation being fractured. Usually a gel, an emulsion or a foam having a proppant such as sand or other particulate material suspended therein is introduced into the fractures. The proppant is deposited in the fractures and functions to hold the fractures open after the pressure is released and the fracturing fluid flows back into the well. The fracturing fluid has a sufficiently high viscosity to retain the proppant in suspension or at least to reduce the tendency of the proppant to settle out of the fracturing fluid as the fracturing fluid flows along the created fracture. Generally, a viscosifier such as a polysaccharide gelling agent is used to gel the fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process.
After the high viscosity fracturing fluid has been pumped into the formation and fracturing of the formation has occurred, it is desirable to remove the fluid from the formation to allow hydrocarbon production through the new fractures. Generally, the removal of the highly viscous fracturing fluid is realized by "breaking" the gel or in other words, by converting the fracturing fluid into a low viscosity fluid. Breaking the gelled fracturing fluid has commonly been accomplished by adding a "breaker," that is, a viscosity-reducing agent, to the fracturing fluid prior to pumping into the subterranean formation. However, this technique can be unreliable and sometimes results in incomplete breaking of the fluid, particularly when gel stabilizers are present, and/or premature breaking of the fluid before the fracturing process is complete.
U.S. Pat. No. 5,413,178 discloses a means for breaking an aqueous based fracturing fluid or treatment fluid at a temperature above 200.degree. F., through use of an effective amount of an alkali metal chlorite or hypochlorite. The breaker is particularly effective in the temperature range of 200.degree. F. to about 300.degree. F. It has been determined that alkali metal chlorites generally are ineffective as a breaker within a reasonable time period at temperatures below about 200.degree. F.
Thus, it is desirable to provide means by which alkali metal chlorites or hyprochlorites can be utilized to provide controlled breaks of viscosified fluids at temperatures in the range of from about 100.degree. F. to about 200.degree. F.